Promoters that regulate gene expression in plants are essential elements of plant genetic engineering. Several examples of promoters useful for the expression of selected genes in plants are now available (Zhu, et al., 1995; Ni, et al., 1995).
To be expressed in a cell, a gene must be operably linked to a promoter which is recognized by certain enzymes in the cell. The 5' non-coding regions of a gene (i.e., regions immediately 5' to the coding region), referred to as promoters or transcriptional regulatory regions, initiate transcription of the gene to produce a mRNA transcript. The mRNA is then translated at the ribosomes of the cell to yield an encoded polypeptide.
Promoters typically contain from about 500-1500 bases, and can provide regulated expression of genes under their control. A promoter used for expressing a heterologous gene in plant cells may be characterized as (i) a constitutive promoter, that is, a promoter capable of causing similar levels of gene expression in all or many plant tissues, or, (ii) a tissue selective promoter, that is, one which is capable of regulating gene expression to select tissues in a plant transformant (e.g., leaves or fruit).
Many such promoters have been characterized, including those derived from plant viruses, Agrobacterium genes, and a variety of plant genes. Considerable effort has gone into the isolation and characterization of constitutive promoters to drive the expression of a variety of heterologous genes in plant systems.
Viral promoters (i.e., promoters from viral genes) for expressing selected genes in plants, have been identified in the caulimovirus family of viruses (a group of double-stranded DNA viruses), and include the Cauliflower Mosaic Virus (CaMV) 35S (Balazs, et al, 1982; Guilley, et al., 1982; Odell, et al., 1985; Odell, et al., 1987; Odell, et al., 1988; Tommerup, et al., 1990; Jefferson, et al., 1987a; Jefferson, 1987b) and CAMV 19S promoters (Fraley, et al., 1994), and the Figwort Mosaic Virus (FMV) (Rogers, 1995) promoter. Promoters useful for regulating gene expression in plants and obtained from bacterial sources, such as Agrobacterium-derived promoters, have been identified and isolated. Such promoters include those derived from Agrobacterium T-DNA opine synthase genes, and include the nopaline synthase (nos) promoter (Rogers, 1991), the octopine synthase (ocs) promoter (Leisner and Gelvin, 1988) and mannopine synthase (mas) promoter.
Plant promoters (promoters derived from plant sources) effective to provide constitutive expression, are less well known, and include hsp80, Heat Shock Protein 80 from cauliflower, (Brunke and Wilson, 1993), and the tomato ubiquitin promoter (Picton, et al., 1993). These promoters can be used to direct the constitutive expression of heterologous nucleic acid sequences in transformed plant tissues. At present, a relatively small number of plant promoters, particularly constitutive plant promoters, has been identified. The use of such promoters in plant genetic engineering has been rather limited to date, since gene expression in plants is, for the most part, typically tissue, developmentally, or environmentally-regulated.